A field effect transistor is widely used as a unit electronic device of a semiconductor memory integrated circuit, a high-frequency signal amplification device, a liquid crystal drive device, or the like and is an electronic device which is most practically used at present.
Among the devices, with remarkable development of displays in recent years, a thin film transistor (TFT) is often used as a switching device for driving a display by applying a drive voltage to a display device in not only a liquid crystal display (LCD) but also various displays such as an electroluminescence display (EL) and a field emission display (FED).
As the material of the thin film transistor, a silicon semiconductor compound is used most widely. Generally, silicon single crystal is used for an RF amplification device, a device for an integrated circuit, and the like requiring high-speed operation. Amorphous silicon is used for a liquid crystal driving device or the like to address a demand for a larger area.
However, at the time of crystallizing a crystalline silicon thin film, a high temperature such as 800° C. or higher is necessary, so that it is difficult to form the crystalline silicon thin film on a glass substrate or an organic substrate. There are consequently problems such that a crystalline silicon thin film can be formed only on an expensive substrate having high heat resistance such as silicon wafer, quartz, or the like, and enormous energy and a number of processes are required for manufacture.
On the other hand, an amorphous silicon semiconductor (amorphous silicon) which can be formed at a relatively low temperature has a switching speed lower than that of a crystalline one. Therefore, when the amorphous silicon semiconductor is used as a switching device for driving a display, there is the case that the semiconductor cannot follow high speed display of a moving picture.
Further, when a semiconductor active layer is irradiated with a visible ray, it becomes conductive. There is the possibility such that a leak current is generated, and an erroneous operation is performed. A problem occurs such that the characteristics of the switching device deteriorate. There is known a method of providing a light shield layer for blocking the visible ray to address the problem. As the light shield layer, for example, a metal thin film is used.
However, when the light shield layer which is a metal thin film is provided, the number of processes increases and, in addition, floating potential is generated. It is consequently necessary to set the light shield layer to the ground level. In this case as well, a problem occurs such that parasitic capacitance is generated.
Since the visible ray transmittance is low, when the semiconductor layer lies off to an electrode part, the transmittance of a display part decreases, illumination efficiency of back light deteriorates, and the screen may become darker. Allowance of process precision is small, and it is a factor of high cost.
At present, as the switching device for driving the display, a device using a silicon semiconductor film is the mainstream for the reasons that various performances of silicon thin films such as high stability, high processibility, and high switching speed are excellent. Such silicon thin films are manufactured generally by chemical vapor deposition (CVD) method.
There is a conventional thin film transistor (TFT) having an inversely-staggered structure in which a gate electrode, a gate insulating layer, a semiconductor layer made of amorphous silicon hydride (a-Si:H) or the like, and source and drain electrodes are stacked on a substrate made of glass or the like. In the field of a large-area device such as an image sensor, the conventional thin film transistor is used as a drive device of a flat panel display or the like typified by a liquid crystal display of an active matrix type. In those uses, as the functions become higher also in a conventional device made of amorphous silicon, higher operation speed is in demand.
Under such circumstances, in recent years, attention is being paid to a transparent semiconductor thin film made of a metallic oxide such as zinc oxide, particularly, a transparent semiconductor thin film made of a zinc oxide crystal as a film having stability higher than that of the silicon semiconductor thin film (amorphous silicon).
For example, a method of constructing a thin film transistor by crystallizing a zinc oxide at high temperature is described in Patent Documents 1 and 2, or the like.    Patent document 1: Japanese Unexamined Patent Application Publication No. 2003-86808    Patent document 2: Japanese Unexamined Patent Application Publication No. 2004-273614